1. Field of the Invention
The present invention applies to the field of wireless communications systems using adaptive antenna arrays and, in particular, to allocating communications channels and terminals using spatial strategies in a spatial division multiplexed wireless communications system.
2. Description of the Prior Art
A typical wireless communications system is generally subdivided into cells. A cell is generally thought of as a distinct geographic area, although cells usually overlap in reality. A cell is generally associated with a base station providing service to user terminals that enter the cell, i.e., the base station's service area. Thus, a cell may also be thought of as a collection of remote terminals communicating with a particular base station at a certain time. Thus, while cells are pictured geographically, it is possible for two user terminals in close proximity to be in different cells, so long as they are communicating with different base stations of the wireless radio network.
Adaptive antenna arrays and SDMA (Spatial Division Multiple Access) enable a wireless system to use strategies to reduce interference and enhance system capacity. These strategies include 1) increasing the signal to interference ratio on the uplink (user terminal to base station) by adjusting received signal samples based on the location of a remote terminal and the RF environment, 2) concentrating signal power to the intended user terminal (beam-forming), and 3) placing nulls to user terminals using similar or the same frequency resources, such as terminals using the same channel on the downlink (base station to user terminal), among others. With these strategies, adaptive arrays can greatly enhance the capacity of a wireless system.
Using various SDMA strategies, as described above, a single base station may be able to communicate with more than one user terminal on the same conventional communications channel. The number of user terminals with which a base station can successfully communicate using a single communications channel varies. It can depend on the number of other user terminals on other channels, the nature of physical obstructions to the radio signals, the amount of RF (radio frequency) noise in the environment, and the design of particular radios and the overall system among other factors.
Prior art SDMA systems have generally used one conventional channel for one user terminal, and used the co-channel interference mitigating capabilities provided by SDMA for mitigating interference to co-channel user terminals in other cells. A co-channel user terminal can be viewed as another user terminal using the same conventional channel. For co-channel users communicating with another base station, in a different cell, the reuse frequency of the channel determines how near the co-channel users are. This affects how much co-channel interference is created. SDMA used to increase reuse frequency can increase the capacity of the system by allowing more aggressive frequency reuse.
As an alternative, the SDMA system can assign a predetermined number of user terminals to each channel. For example, three user terminals using SDMA could share each channel. Unfortunately, a system with a predetermined number of user terminals is unlikely to operate at maximum capacity, or to provide optimum quality of service. The number of co-spatial users that can successfully share a channel typically depends on the individual characteristics of each user terminal and its reception on its assigned channel. In most real systems, some user terminals will be able to successfully share a channel with more co-spatial users than others.
Therefore, using a predetermined number for co-spatial terminals will likely result in some channels being overused and some being underused. Underused channels waste capacity, and overused channels may have an unacceptably low quality of service. In order to ensure high quality service on all channels, some channels will be underused. This reduces the capacity benefits that an SDMA system can offer.